1. Field of the Invention
The present invention relates to an evaporator, and more particularly, to an evaporator used for a refrigeration cycle in an air conditioning apparatus for a vehicle.
2. Description of the Related Art
An air conditioning apparatus for a vehicle is generally constructed by a compressor, a condenser, a receiver, an expansion valve and an evaporator, which are located on a recirculation line of a cooling medium for obtaining a refrigeration cycle, and a heat exchange takes place in the condenser with respect to an air directed to a cabin. Namely, a cooling medium from the condenser is expanded at the expansion valve, to obtain a combined gas and liquid state thereof, and then introduced into an evaporator, whereby the gas is evaporated by absorbing heat from the outside while still subjected to the isothermal expansion process, to thereby cool air flowing to the cabin. The evaporated and heated gas is introduced into the compressor under a superheated vaporized condition.
A stacked type evaporator is known, wherein a plurality of evaporating portions are stacked and a common inlet for the refrigerant is provided for dividing the flow of the refrigerant from the inlet into a plurality of refrigerant passageways, to thus obtain a flow of the refrigerant in each of the respective passageways. At the inlet of this type of evaporator, the refrigerant is in a combined air-liquid state, and therefore, it is difficult to evenly distribute the refrigerant to the passageways, and thus the heat exchange efficiency is reduced. To combat this problem, Japanese Examined Patent Publication No. 58-41429 proposed a stacked type condenser, wherein a fixed restriction is provided at each evaporating portion, to thereby allow the liquid state refrigerant condensed and liquidized at the condenser to be introduced into the respective passageways without passing through an expansion valve, to thus obtain an even distribution of the refrigerant.
The refrigerant must be completely gasified at the evaporator, to prevent a liquid state compression at the compressor, and accordingly, a heat exchange at the evaporator is carried out such that the refrigerant is brought to a superheated condition of a predetermined degree of superheat at the outlet of the evaporator. As is well known, the temperature of the refrigerant remains the same during the evaporation, but a superheated vapor condition after the completion of the evaporation can cause an increase in the temperature of the refrigerant, and as a result, a temperature difference will be created across the evaporator between the evaporator inlet at which the refrigerant is evaporating and the evaporator outlet at which the refrigerant is in a super-evaporated state. Such a temperature difference causes the air to be unevenly cooled at different areas of the evaporator, which makes the passenger feel uncomfortable. To combat this difficulty, the Journal of Nippon Denso Technical Disclosures No. 40-076, published in Japan on Mar. 15, 1985, discloses a refrigerating system wherein a high temperature conduit upstream of an expansion valve and a low temperature conduit downstream of the evaporator, at a position between the evaporator and a thermo-sensitive tube, are arranged so that a heat exchange takes place therebetween to thus suppress the superheat generated in the evaporator.
The stacked type evaporator disclosed in Japanese Examined Patent Publication No. 58-41429 suffers from a drawback in that, due to a high pressure inside a tank of the evaporator, the various parts constructing the evaporator must have a high pressure resistance, making the system large in size overall. Further, in this prior art, the evaporator directly receives the liquid state refrigerant as liquidized at the condenser, and no provision is made for controlling the evaporation of the liquid state refrigerant, and thus the system cannot quickly respond to changes in the air conditioning load.
The refrigerating system provided for obtaining a heat exchange between the high temperature conduit upstream of the expansion valve and the low temperature conduit downstream of the evaporator, as disclosed in the Journal of Nippon Denso Technical Disclosures No. 40-076, makes the entire refrigerating system complicated. Also, the direct introduction of the liquid refrigerant condensed at the evaporator lacks a means for a control of the evaporation process, and this makes it difficult to cope with changes in the air conditioning load.
Conversely, the refrigerating system wherein a heat exchange is carried out between the high temperature conduit upstream of the expansion valve and the low temperature conduit downstream of the evaporator also makes the total system complicated. Further, a control of the heat exchange between the conduits is apt to be unstable due to time delays.